Electroluminescent (EL) materials most often emit light in the visible or infrared spectrums when electrically stimulated. Common electroluminescent materials include, for example, zinc sulfide, calcium sulfide, strontium sulfide and various phosphor compounds such as phosphorous or zinc sulfate. Such materials are frequently used in various light sources, particularly in electronic displays such as, for example, flat panel displays, helmet-mounted displays, and heads-up displays. Although sulfides and phosphors provide adequate light for some applications, many of these substances are frequently susceptible to hydrolysis that may destabilize the light source or degrade the source's effective life. Moreover, many sulfides and phosphors exhibit undesirable thermal properties in that their thermal coefficients of expansion (TCE) frequently do not match those of many common display device packaging materials. With regard to manufacturability of these devices, this incompatibility often leads to low device yields due to narrow processing margins.
Adding rare earth elements such as, for example, erbium (Er), ytterbium (Yt), praseodymium (Pr) or cerium (Ce) to the electroluminescent material frequently enhances the light generating properties of an EL substance. Rare earth atoms are believed to produce light as electrons at high energy states release photons as they transition to lower energy states. To generate light, electrons in rare earth atoms must be "excited" from their natural energy state to a higher state. Excitation may be accomplished by any excitation method, such as, for example, electrical stimulation or "pumping" by an external light.
Erbium, for example, can be excited from the I.sub.15/2 energy state to the I.sub.13/2 state by shining a pump light having a wavelength of approximately 980 =l nm on the Erbium atoms. Different rare earth elements and different energy levels for each rare earth element require different frequencies of "pump" light to excite electrons into various states of excitation. After the rare earth element is excited, light may be generated by stimulating the excited atoms back to their original energy states by light of a second wavelength that is typically selected according to the particular energy state and particular rare earth element. To stimulate Erbium from the excited I.sub.13/2 state to the I.sub.15/2 state, for example, a stimulant light having a wavelength of about 1550 nm could be used. The light emitted will have a wavelength approximately equal to the wavelength of the stimulant light. Alternatively, electrons can be pumped and stimulated by electrical energy, as described herein. Typically, rare earth elements may be inserted into an EL substance by a heat injection method that typically involves heating a substrate material to a very high temperature (typically on the order of 400 degrees Celsius) and then injecting rare earth atoms into the molten substrate. Rare earth elements frequently tend to bond together (i.e., "cluster") at high temperatures, and therefore the heat injection method frequently results in uneven distribution of rare earth elements throughout the substrate. The uneven distribution typically results in degraded overall performance of the light source, because certain regions of the EL material substantially lack light-emitting rare earth atoms. Therefore, the heat injection method frequently results in sub-optimal quantities of light generated by the electroluminescent device. Clustering also frequently tends to decrease the amount of light generated because rare earth atoms that are closely clustered may absorb photons emitted by other atoms in the cluster, thus often reducing the total amount of light produced.
Several others have attempted to create rare-earth based EM materials, usually with disappointing results. For example, the article "Erbium-doped Indium Oxide Films Prepared by Radio Frequency Sputtering", written by Hong Koo Kim, et al, and published in the November/December 1994 edition of the Journal of Vacuum Science, which is herein incorporated by reference, generally discloses an attempt to create a light source by doping indium oxide with erbium in an RF sputtering environment. As noted by Kim, et al, conducting oxides such as InO may exhibit several beneficial light generating properties in that they generally have a wide band gap and are relatively easy to dope with rare earth elements. Kim, et al, note, however, that high levels of doping typically results in degraded crystallinity and therefore poor electrical properties. Indeed, the doped indium oxide material disclosed by the Kim, et al, reference allegedly produced some light, but not enough to be beneficial in a practical light source.
U.S. Pat. No. 4,027,192 issued to Joseph John Hanak on May 31, 1977, which is herein incorporated by reference, generally describes a display based upon a conducting oxide phosphor that also includes Indium Tin Oxide. Rare earth atoms are RF sputtered into the phosphor to adjust the resistance of the phosphor, which is stated to be on the order of 10.sup.8 -10.sup.10 .OMEGA.-cm. Although this display may be capable of generating some light, it retains a reliance on phosphor compounds that are frequently expensive and often difficult to manufacture. Similarly, U.S. Pat. No. 5,543,237 issued to Masao Watanabe on Aug. 6, 1996, the entire contents of which are herein incorporated by reference, generally discloses a light source for a flat panel display that includes a light emitting layer that is made up of an alkaline earth metal such as calcium, magnesium or barium doped with a rare earth element through a vapor deposition process.
It is therefore desirable to produce an electroluminescent light source that has improved environmental robustness (and therefore a longer life span) than sulfide light sources. It is also desirable to inject the source material with a high degree of rare earth dopant while minimizing clustering. Moreover, it is beneficial to produce a light source that is substantially transparent for use in flat panel, helmet-mounted or heads-up displays.